Nuclear power
Nuclear power plants provide about 17% of the world's electricity. Pros

Nuclear power can generate large quantities of energy without releasing greenhouse gases

It does not depend on the weather

The UK's largest reactor generates the equivalent output of 1188 wind turbines. A nuclear fuel pellet, about half an inch long, provides the same amount of electricity as one and a half tonnes of coal.

Cons

Spent fuel from nuclear power plants remains toxic for centuries and there is no safe permanent storage facility for it

Dismantling old reactors safely is highly expensive

Mining and enriching uranium also produces toxic waste

Uranium is a finite resource, although there are ways of recycling spent fuel

Fears of nuclear material stolen from power facilities being used by terrorists in a so-called "dirty bomb" have increased since the 11 September attacks

Nuclear power is currently generated from nuclear fission - splitting the nuclei of atoms. Nuclear fusion - combining atomic nuclei - is potentially cleaner because the fuels involved are different. But scientists have been trying to harness the energy from fusion for decades and some say a working fusion generator is still a lifetime away.

There is a close link betweeen civil and military uses of nuclear power

How it works
A nuclear reaction is a change in the nucleus of an atom, and can be triggered by bombarding an atom with a subatomic particle, such as a neutron or proton. In a nuclear fission reactor, a neutron strikes the nucleus of an atom with a heavy nucleus, such as a particular type - or isotope - of uranium atom. The nucleus of this atom splits, releasing huge amounts of energy, mainly in the form of heat. Other neutrons are also released, which initiate a chain reaction by striking other nuclei. Nuclear reactors control this reaction and use the heat to generate electricity.

Nuclear fusion - also the reaction by which the Sun generates heat - is still in development. Light nuclei, such as those of isotopes of hydrogen, are combined to make heavier ones, such as helium, at very high temperatures and pressures. Large amounts of energy are released in the process but the reaction has proved to be difficult to contain and control.

Nuclear fission power plants provide about 75% of electricity in France, a quarter in the UK and 15% in the US. More than 100 of the world's 400 nuclear power plants are in the US.